If a vehicle was always driven in a straight path without having to make any turns, differentials would not be required, however, when an automobile makes a turn, the outside drive wheel must travel significantly farther than the inside wheel, thus requiring the outside wheel to rotate at a higher rate of speed than the inside wheel. A differential solves this problem by allowing the outside and inside wheels to rotate at the same rate when the vehicle is traveling straight ahead, but at a different rate when turns are made. Most standard differentials have been designed to deliver equal amounts of torque to each driven wheel; however, if one of the wheels loses traction, e.g., slips on ice, then the other wheel cannot deliver torque. Accordingly, many cars are now equipped with devices to prevent this problem.
One of these devices, the limited slip differential, is similar in construction to the standard differential, but, in addition, it uses clutch plates to prevent a wheel which has lost traction from spinning wildly. These clutch plates restrain the spinning axle and as a result, if one wheel loses traction temporarily, the other will not spin out of control, thereby ensuring that torque is still supplied to the non-spinning wheel.
Although the limited slip differential has served a purpose, it has not proven to be satisfactory under all conditions. For example, a small amount of slippage occurs between the time one of the wheels begins to slip and before the clutch plates lock onto the axle. As a result, the driver momentarily loses optimal control of the vehicle. Another problem which occurs is that a limited slip differential tends to wear out over time, thereby reducing its effectiveness and requiring costly replacement. Finally, the complexity of the limited slip differential adds significantly to the purchase and maintenance costs of a vehicle.
A second attempt to tackle this problem has resulted in the development of traction control systems which utilize on-board computers. With these systems, when slippage occurs at one of the drive wheels, a signal is sent to the system's computer. Reacting to the signal, the computer activates a brake on the slipping wheels, thereby enabling the companion wheel to transmit torque. The problem with this arrangement is that slippage must occur before the system is activated. As a result, during the most critical point of the operation, the point at which slippage occurs, the system does not operate.
Another attempt to solve the problems encountered with the limited slip differentials is seen in the Torsen drive. The configuration of the Torsen drive, consisting substantially of worm gears, ensures that sufficient power is delivered to each wheel at all times. For straight-ahead operation, on surfaces having equal coefficients of friction, torque is delivered to each wheel at a 50-50 ratio. As traction is lost, the wheels are provided power in proportion to their ability to grip the road. Although differentials of the Torsen drive-type have proven to operate effectively, the complexity of the gears and its expense have discouraged commercial use.
A final method for controlling loss of traction is with the locking differential, however, its operating characteristics are such that it has very little usage.